1. Field of the Invention
The present invention relates to a method of manufacturing a sintered compact of cubic boron nitride employed as a tool or a heat sink material for example, and more particularly, it relates to a method of manufacturing a sintered compact of cubic boron nitride, hereinafter referred to as a cBN sintered compact, from hexagonal boron nitride, hereinafter referred to as hBN.
2. Description of the Prior Art
The cBN has a hardness that is almost the same as the hardness of diamond. While diamond is oxidized and exhausts upon reaction with iron family elements when heated at a temperature within a range of 700.degree. to 800.degree. C. or more, cBN is not oxidized, hardly converted into hBN and does not react with any member of the iron family even under a high temperature in the range of 1100.degree. to 1300.degree. C. Thus, cBN is chemically and thermally extremely stable, and suitable as a material for grinding a member of high-speed steel, alloy or cast iron having a substrate of nickel or cobalt etc., which cannot be ground by diamond. Further, cBN is next to diamond and by far higher than copper, in heat conductivity, whereby the same is also suitable for a heat sink material. A cBN sintered compact for the aforementioned usage is preferably of high purity and high density with no admixture or with an extremely small amount of admixture.
However, it is extremely difficult to directly sinter cBN powder and hence metal, carbide, oxide or nitride must be generally admixed to the same as a binder, whereby the sintered compact is made non-uniform in its structure and its mechanical strength and thermal stability is extremely reduced, while the heat conductivity is remarkably lowered.
In order to solve the aforementioned problems, the following cBN sintering methods are known:
U.S. Pat. No. 3,212,852 and Japanese Patent Publication Gazette No. 13731/1966 discloses a method of directly converting an hBN material into cBN without admixing a catalyst or binder, to obtain a dense cBN sintered compact. However, this method requires superhigh pressure and temperature conditions in excess of 100 Kbar and 2000.degree. C., and the product obtained is considerably restricted in configuration. Japanese Patent Publication Gazette No. 2625/1986 also discloses a method of directly converting hBN into cBN, which method requires operation under a high temperature in excess of 1600.degree. C., leading to difficulty in production.
On the other hand, U.S. Pat. Nos. 4,188,194 and 2,947,617 and Japanese Patent Laying-Open Gazette No. 33510/1979 disclose a method of directly converting a material of pyrolytic boron nitride (pBN) into cBN, thereby to obtain a cBN sintered compact. However, pBN is still partially unknown in structure and the same is prepared by chemical vapor deposition through thermal decomposition of BCl.sub.3 and NH gas, whereby the cost is increased. Further, this method requires high-temperature processing under a temperature in excess of 1800.degree. C. as described in Japanese Patent Laying-Open Gazette No. 33510/1979, leading to a problem in manufacturing.
Japanese Patent Laying-Open Gazette No. 128700/1976 discloses a method of directly converting a material of wurtzite boron nitride, hereinafter referred to as wBN, which is a high-pressure phase BN, into a cBN sintered compact. However, since wBN is generally in a thermodynamically unstable, non-equilibrium state, the state of the material must be controlled with a high accuracy. Such strict control of the material state is extremely difficult in practice, and hence hBN or wBN is deposited in the obtained cBN sintered compact whereby the product quality becomes irregular.
On the other hand, a method of manufacturing a cBN sintered compact containing no bonding phase with relatively relaxed conditions at a low cost is described in, for example, a report by Wakatsuki et al. in "Material Research Bulletin", Vol. 7, 1972, p. 999. In this method, a cBN sintered compact is obtained by directly converting a parent material of low-crystalline hexagonal boron nitride hBN. However, the low-crystalline hBN employed as the parent material is chemically unstable and it reacts easily with oxygen in the air, and hence it is difficult to obtain a compact uniformly and sufficiently sintered as a whole.
It is well known in the art that cBN can be obtained by superhigh pressure and temperature processing of an hBN material under presence of a catalyst. Typical examples of such a catalyst are alkaline metal, alkaline earth metal and nitride thereof. A method of synthesizing cBN from hBN in a relatively low temperature condition with a catalyst of water is described in "Material Research Bulletin", Vol. 9, 1974, p. 1443, Susa et al. In this method employing water as a catalyst, however, a dense sintered compact having interparticle bonding cannot be obtained since borate is generated and the cBN is extremely small in particle size.
Japanese Patent Publication Nos. 38164/1984 and 38165/1984 disclose alkaline earth metal boron nitride such as magnesium boron nitride, strontium boron nitride and barium boron nitride as catalysts for converting hBN into cBN. Further, Japanese Patent Publication No. 5547/1984 and Japanese Patent Laying-Open No. 57967/1984 describe a method of performing a superhigh pressure and temperature processing on an hBN sintered compact carrying a small amount of alkaline earth metal boron nitride as disclosed in Japanese Patent Laying-Open No. 57966/1984 at a temperature in excess of 1350.degree. C. However, this method requires high-temperature operation at a temperature in excess of 1350.degree. C., and hence considerably high temperature processing is required in order to internally homogeneously sinter a large-sized sample. If the temperature control is incorrectly performed at such high temperature processing, unsintered parts are left in the sintered compact. Thus, it has been difficult to obtain a dense sintered compact.